1. Field of the Invention
This invention relates to a color image processor, and for example more particularly to a color image processor having a masking function for masking a digital color image signal.
2. Description of the Related Art
Hitherto, color correcting masking processes have been employed in this kind of system. As for this masking process, the following linear masking equations shown as (1) through (3) corresponding to each color signal of yellow (Y), magenta (M) and cyan (C), are often adopted: EQU Y.sub.0 =a.sub.11 Yi+a.sub.12 Mi+a.sub.13 Ci (1) EQU M.sub.0 =a.sub.21 Yi+a.sub.22 Mi+a.sub.23 Ci (2) EQU C.sub.0 =a.sub.31 Yi+a.sub.32 Mi+a.sub.33 Ci (3)
There is a table-referring method for realizing these equations.
FIG. 1 shows a block diagram of the main components for explaining the masking method of the prior art. In the referring method of the prior art, the output image data, Yo, Mo and Co, with the unnecessary color components deleted, are stored in advance in the table corresponding to the combinations of the concentrations of the digital color image signals, Yi, Mi and Ci, which are inputted to the masking table 100 in this figure, and the output image data, Yo, Mo and Co, corresponding to the combination addresses of the color image signals, Yi, Mi and Ci, which are inputted at the time of image processing, are read out from the masking table 100 and are outputted.
The masking table 100 shown in FIG. 1 is taken from the prior art, and the input color image signals, Yi, Mi and Ci, re represented as 8-bit signals for each 1 picture element, and at the same time the output image signals, Yo, Mo and Co, are also represented as 8-bit signals.
However, in the above described example of the prior art, a memory capacity of 3.times.2.sup.24 bytes (48 megabytes) is required to obtain the output image data, Yo, Mo and Co, since the combination addresses of the input color image signals Yi, Mi and Ci are 2.sup.24 and each corresponding output contains the output of 3 colors consisting of 1 byte (8 bits) each.
Thus, a disadvantage of the table-referring method of the prior art is that an enormous memory capacity is required to perform accurate masking processes.
Alternatively, the following method can be used to reduce the size of the masking table as above mentioned. In this method, the color corrected signals in each color at the time of output are obtained by using the upper-order M bits in the N bits (N&gt;M) of digital color image signal as the input address when the masking process is performed on the N-bits digital color image signal which is separated into a plurality of colors.
In this method, as shown in FIG. 2, a difficult masking table 200, instead of the masking table 100 of the prior art is used. The upper-order 6 bits of the input color image signal of 8-bit data are used for the address data inputted to the masking table 200, and the lower-order 2 bits are eliminated. In this case, the number of the combination addresses of the input color image signals, Yi, Mi and Ci, is 2.sup.18, and the required memory capacity is 3.times.2.sup.18 bytes (768 kilobytes). Therefore, the memory requirements can be drastically reduced in this way, avoiding a masking table using a full 3.times.8-bit address.
Now, the method for reducing the input color image signal from 8-bits to 6-bits will be explained. In this method, as shown in FIG. 3, the data of 8-bits can be shifted to the right by two bits. However, it is unavoidable that the accuracy of the data is reduced by reducing the data by up to three pieces for each two bits deleted. Thus, when the above simple method to discard the lower-order 2 bits of 8-bit data is employed, the data of photographic density having 256 gradations are forcibly reduced to 64 gradations and the density expressing ability at high gradation is lost accordingly.